1. Field of the Invention
The present invention relates to a maintenance dedicated line at a signal transmission system, so-called an orderwire.
2. Description of the Related Art
An orderwire is a maintenance dedicated line of a signal transmission apparatus, and is provided, for example, at a manhole for an under ground cable, and the like, usually as a station having no interchanging function, for the purpose that an operator, who mainly performs a maintenance operation, uses it.
FIG. 1 is a schematic diagram showing such general orderwire line,
In FIG. 1, reference characters SA, SB, SC, SD and SE designate stations respectively having orderwire functions, and the respective inherent calling numbers "111", "222", "333", "444" and "555" are assumed to be set in advance.
In addition, in FIG. 1, telephones TELl, TEL2, TEL3, TEL4 and TEL5 are connected respectively to the stations SA, SB, SC, SD and SE, however, these telephones TELl, TEL2, TEL3, TEL4 and TEL5, in reality, are used by an operator who performs maintenance operation every time at need by connecting his (or her) portable telephone to any of the station SA, SB, SC, SD or SE.
The convenience of explanation, in the following drawings, the left side is called to be W side and the right side to be E side.
FIG. 2 is a block diagram showing a configuration of a conventional apparatus for the orderwire line of each station SA, SB, SC, SD and SE.
In FIG. 2, a signal line L1 is inputted from the W side to the station S and outputted to the E side through a demultiplexing circuit (DMUX) 11W which demultiplexes a multiplexed signal, a codec circuit 13W which performs analog/digital conversion or digital/analog conversion, an adding unit A1, a codec circuit 13E, and a multiplexing circuit (MUX) 12E.
On the other hand, in FIG. 2, the signal line L2 is inputted from the E side to the station S and outputted to the W side through a demultiplexing circuit 11E, the codec circuit 13E, an adding unit A2, the codec circuit 13W, and multiplexing circuit 12W.
Reference character A3 designates an adding unit, and a signal is inputted thereto from a portion between the codec circuit 13W and the adding unit Alon the signal line L1 and a portion between the codec circuit 13E and the adding unit A2 on the signal line L2, and adds them to output to a station number detecting circuit 14.
The station number detecting circuit 14 is specifically a DTMF (Dial Tone Multi-Frequency) receiver, which distinguishes the DTMF tone of a push button type telephone, thereby detecting a station number from a signal obtained by adding the two signals inputted from the two signal lines L1 and L2.
Reference numeral 15 designates a station number coincidence detecting and call in operating unit which performs a call in operation when it detects that a station number detected by the station number detecting circuit 14 coincidence with the station number set in itself.
Reference numeral 16 designates a telephone interface (TEL I/F), which outputs a signal inputted from the both signal lines L1 and L2 to the telephone TEL through the adding unit A3 and the station number detecting circuit 14, and outputs a signal inputted from a telephone TEL to the signal line L1 after adding it at the adding unit A1 and to the signal line L2 after adding at the adding unit A2.
As described above, since the orderwire line has no interchanging function, the stations SA, SB, SC, SD and SE shown in FIG. 1 are connected to the same line. In the case where a telephone communication is performed with such orderwire line, when an operator calls out by connecting the telephone TEL to a certain station and by pushing the pushing button, the DTMF tone is transmitted to the other stations. The station of the call in side judges whether the calling is for itself or not, and when it is the calling for itself, a call in operation is performed to inform the operator.
Accordingly, when a communication is performed by using the orderwire line between the stations SA and SB, as shown in FIG. 1, since the signal inputted to the telephone TEL is obtained by adding signals from the signal lines L1 and L2 of both sides at the adding unit A3 similarly to be interfaced by the TEL I/F 16, as shown in FIG. 2, when a communication is performed between the station SC or SD, a cross talk is caused between the stations SA and SB.
Therefore, in order to enable a communication also between the stations SC and SD at the same time of a communication between the stations SA and SB by using the orderwire line, the line between the stations SB and SC is cut-off only with respect to the orderwire line, by time slot interchanging in advance. By such time slot interchanging, a communication between the stations SC and SD due to the orderwire line also becomes possible at the same time of a communication between the stations SA and SB due to the orderwire lines.
By the way, in order to perform communications between the stations SA and SB, and the stations SC and SD, it is necessary to perform time slot interchanging in advance by same means. When such a time slot interchanging is performed, a communication between the stations SA and SC or SD, or that between the stations SB and SC or SD cannot be done by the orderwire line, it is necessary to cope with the situation by performing a different time slot interchanging.
Meanwhile, in the SDH (Synchronized Digital Hierarchy) which is a network for using the ISDN (Integrated Services Digital Network) practically, 2 bytes (E1 and E2 byte) in a frame which has transmitting ability of 64 K bit/second per one byte are provided for the orderwire.
FIG. 3 is a schematic diagram showing a frame format of such SDH.
One frame is composed of 1080 bytes (90 bytes.times.12 multiplex).times.9 rows, and it is roughly composed of an overhead portion which is a header for a frame synchronizing signal and data for transmitting various auxiliary signals, and of a pay-load portion for transmitting information data. In addition, a frame repeating cycle, that is, one frame cycle is 125 .mu.s.
The overhead portion includes all of the various signals necessary for transmitting multiplexed signals such as frame synchronizing signals, error supervising codes, channel discriminating signal, channel for maintenance, alarm signal, and the like.
Each one byte of E1 and E2 is allocated to one multiframe for the use of the orderwire in the overhead portion, and other control signals are not included. Therefore, in the transmission system, for example, in which transmission apparatuses DA, DB, DC, DD and DE shown in FIG. 4 are connected by optical fibers, in order to perform a communication between all of the transmission apparatuses DA, DB, DC, DD and DE by the orderwire, the configuration of each transmission apparatus is needed to be the one as shown in a block diagram of FIG. 5.
In FIG. 5, an optical signal inputted from an optical fiber F1W at the W side is inputted to an overhead data extracting circuit 23W through an O/E converter 21W which performs optical/electric (O/E) conversion and a demultiplexing circuit (DMUX) 22W which demultiplexes a multiplexed signal, to be extracted only of the overhead portion thereof. The E1 and E2 bytes of the overhead portion are converted into analog signals by a codec circuit 31W of an orderwire outside interface unit 50.
In FIG. 5, an optical signal inputted from an optical fiber F1E at the E side is inputted to an overhead data extracting circuit 23E through an O/E converter 21E which performs optical/electric (O/E) conversion and a demultiplexing circuit (DMUX) 22E which demultiplexes a multiplexed signal, to be extracted only of the overhead portion thereof. The E1 and E2 bytes of the overhead portion are converted into analog signals by a codec circuit 31E of the orderwire outside interface unit 50.
The analog signals outputted from the two codec circuits 31W and 31E are added at an adding unit 33 and outputted to a terminal equipment TER such as a telephone from an orderwire analog interface 38. On the contrary, an input from the terminal equipment TER such as a telephone is branched to the two codec circuits 31W and 31E at the branching unit 37. One branched signal is added to an analog signal outputted from the codec circuit 31E and branched at a branching unit 32 at the adding unit 36, to be given to the codec circuit 31W. The other branched signal is added to an analog signal outputted from the codec circuit 31W and branched at the branching unit 35 at the adding unit 34 to be given to the codec circuit 31E. At the codec circuit 31W, the E1 and E2 bytes of analog signal are converted into digital signals to be given to an overhead data inserting circuit 24W. At the codec circuit 31E, the E1 and E2 bytes of an analog signal are converted into digital signals to be given to an overhead data inserting circuit 24E.
At the two overhead data inserting circuits 24W and 24E, the E1 and E2 bytes are inserted into overhead data and further to main signals, time-division multiplexed by multiplexing circuits (MUXs) 25W and 25E, converted into optical signals by E/O converters 26W and 26E to be outputted to the optical fibers F2W and F2E.
In such a configuration as abovementioned, when a communication is performed by the orderwire between, for example, the transmission apparatuses DB and DD, shown in FIG. 4, it is necessary for the transmission apparatus DC to convert a digital signal into an analog signal, and further to convert it to a digital signal again.
Therefore, as shown, for example, in FIG. 6, such a configuration as to use digital interfaces 39W and 39E as the interfaces for the orderwire can also be thought of. In this case, it is necessary for the interfaces 39W and 39E of a transmission apparatus which does not input/output to perform digital-through, however, the control signals therefor are not prepared in the overhead portion thereof, resulting in setting them separately.
By the way, in the SDH, besides using the orderwire for the original maintenance operation by an sound signal, utilizing the orderwire for data communication by connecting a modem thereto, further utilizing the whole 64K bits for data communication, are increased recently.
For data communication, the maximum problem is a signal error on the way of transmitting a signal. Explanation will be given on the case, for example, when data communication is performed between the transmission apparatuses DB and DD shown in FIG. 4.
The data transmitted from the transmission apparatus DB reaches the transmission apparatus DD via the transmission apparatus DC. At that time, in the case where the transmission apparatus DC has a conventional configuration shown in FIG. 5, when an operator at the transmission apparatus DC uses the orderwire by mistake, the same orderwire lines are accessed to cause a crosstalk because control signals are not set there, resulting in a data error.
In this case, where the transmission apparatus has a configuration shown in FIG. 6, a data error is not generated when digital through is possible, however, setting therefor in advance is necessary. It is also necessary to distinguish whether or not the orderwire line is used by the other operator, but a problem is that it is difficult to do so.
As mentioned above, since only one line is provided to a maintenance dedicated line of a conventional signal transmission apparatus, i.e. an orderwire, in order to perform communications at plural sections of the line, it is necessary to time slot interchanging by some means.
In the orderwire of the SDH, an intermediate transmission apparatus of the communication section must convert a digital signal into an analog signal, and further to a digital signal again. Also when digital interfaces are used as the ones for the orderwire, it is necessary for interfaces of a transmission apparatus which does not input/output to perform digital through, however, control signals therefor are not prepared in the over head portion of a frame, resulting in setting them separately.
Further, a data error is not caused when digital interfaces are used, but advance setting therefor is necessary, and is it also necessary to distinguish whether or not the orderwire line is used by the other operator, but the problem is that it is difficult to do so.